A water absorbent resin is widely used for various purposes, e.g., sanitary materials such as disposable diapers, sanitary napkins, adult incontinence pads, and soil water retaining agents or similar agents, and a great amount of water absorbent resins are produced and consumed. Recently, particularly in the sanitary material purposes such as the disposable diapers, the sanitary napkins, the adult incontinence pads, and the like, there is such tendency that an amount of water absorbent resin is increased and an amount of pulp fiber is decreased to make the product thinner. Thus, the water absorbent resin is required to have a greater absorbency against pressure. While, due to a larger amount of water absorbent resin for each sanitary article, the cost of the water absorbent resin is required to be lower.
Generally, the water absorbent resin is formed by drying and pulverizing a polymer hydrogel, obtained by carrying out aqueous solution polymerization with respect to an unsaturated monomer, into a powdery form. The polymer hydrogel is obtained as a clump or an agglomerate of hydrogel particles. Generally, the polymer hydrogel is roughly crushed by a crusher such as a kneader, a meat chopper, and the like. Further, the roughly crushed hydrogel is dried so that its solid content is 95 weight %, and then the dried hydrogel is pulverized by a pulverizer so as to have a particle diameter corresponding to the purpose or use. Particularly, in case of making a sanitary material for a disposable diaper thinner, it is necessary to use a water absorbent resin having both capillary absorbency and liquid permeability in order to maximize the performance of the absorbent core. Thus, it is known that it is important to control a particle diameter distribution represented by a logarithmic standard deviation within a specific range by setting a mass average particle diameter of the particulate water absorbent resin to be 234 to 394 μm (see Japanese Unexamined Patent Publication No. 261797/2004 (Tokukai 2004-261797)).
However, in the pulverization step after the drying, not only particles whose particle diameter is within the desired particle diameter range but also particles whose particle diameter deviates from the desired particle diameter range occur. For example, if so-called fine powder such as fine particles whose particle diameter is smaller than 150 μm or ultra-fine particles whose particle diameter is smaller than 106 μm is included in the particulate water absorbent resin, this results in lower performance such as lower liquid permeability of the particulate water absorbent resin and lower absorbency against pressure. In this way, inclusion of the fine powder is not preferable. However, the pulverization that causes the mass average particle diameter to be substantially the same as the particle diameter range of the fine particles may increase also an amount of fine particles whose particle diameter is not more than 150 μm. Thus, it is necessary to remove the fine powder, whose amount is relatively large, through classification.
Thus, particles of the pulverized particulate water absorbent resin are sieved with a classifier so as to obtain particles having the aforementioned mass average particle diameter and the aforementioned particle diameter distribution for example. Examples of general classification include: classification using a screen and gravity (classification by sieving); classification using an aerial current and buoyancy (classification by wind force); and the like. In classification of fine particles whose particle diameter is not more than 300 μm for example, the classification by wind force is generally regarded as being suitable (Patent Document 1).
Generally, in order to prevent drop of the yield, fine powder removed by the classification is reformed into larger particles through granulation or the like and then the particles are collected into a production line.
[Patent Document 1]
Japanese Unexamined Patent Publication No. 156299/1999 (Tokukaihei 11-156299)(Publication date: Jun. 15, 1999)